The present invention relates generally to a high-density flash memory, and more particularly to a method to improve the edge breakdown caused by edge electrical field at the tunnel oxide of a high-density flash memory by a shielded bird""s beak.
The development trend of the fabrication technology for integrated circuit (IC) substantially concentrates on the shrinkage of the transistors thereof, by which the switching speed and power consumption could be improved, as well as the integration density and functionality are also enhanced. In a conventional localized oxidation of silicon (LOCOS) method, the capability of miniaturization thereof is limited to the extent of a few microns by the bird""s beak effect and the uneven surface. Therefore, for the purpose of overcoming this problem, shallow trench isolation (STI) technology has been developed for breaking through the bottleneck being confronted.
Typically, the STI process comprises forming a mask on a silicon substrate, defining a STI pattern by lithography process and then etching trenches using the mask, afterward, filling the trenches with an oxide and optionally forming a linear oxide on the trenches prior to the filling the oxide. It is generally followed by a further chemical-mechanical polishing (CMP) for etching-back the oxide and planarizing the surface thereof. Then, the transistor structure could be fabricated on the active region between the STIs.
Although the element with reduced size may be obtained by STI technology, the excessively sharp shape of the corners of the tunnel oxide layer formed later on the active regions may lead to a localized intense electrical field, and thereby resulting in breakdown caused by edge electrical field formed thereof.
Therefore, it is desired a method for improvement of the edge breakdown caused by edge electrical field at the tunnel oxide of a high-density flash memory.
It is accordingly an object of the present invention to propose a method to avoid or reduce the edge breakdown caused by edge electrical field at the tunnel oxide of a high-density flash memory by a shielded bird""s beak.
To improve the edge breakdown caused by edge electrical field at a tunnel oxide of a high-density flash memory, according to the present invention, the method comprises depositing a pad layer on a substrate and then defining a plurality of shallow trench isolations and a plurality of active regions, etching the substrate with the pad layer as a mask to form a plurality of trenches, growing a liner oxide on the trenches to extend to the edges of the active regions for the fabrication of initial bird""s beaks, afterward, depositing an oxide to fill in the trenches, etching-back the oxide by chemical-mechanical polishing with the pad layer as an etch-stop layer to leave a planarized surface, depositing an oxide again to cover the STI oxide after the pad layer is removed, and etching the oxide for formation of bird""s beak spacers to shield the initial bird""s beaks, which will prevent the corners of the tunnel oxide layer formed later on the active regions from being excessively sharp.